Researchers at the University of California, Berkeley and Duke University have shown how to use a single electrical signal to command a group of microrobots to self-assemble into large, complex shapes. The researchers hope to use this method to build biological tissues. But for microrobots to do anything like that, researchers must first figure out a good way to control them.
Bruce Donald, a professor of computer science and biochemistry at Duke, developed a microrobot that responds to electrostatic charges and is activated with voltage through an electric-array surface. Now he and others have demonstrated that they are able to control a group of these microrobots to create complex patterns. They do this by tweaking the design of each robot a little so that each one responds to the same amount of voltage with a different action, resulting in complex behaviors by the swarm.
"A good analogy is that we have multiple control cars but only one transmitter," says Igor Paprotny, a computer scientist at UC Berkeley and one of the lead researchers on this work, which he presented last week at a talk at Harvard University. During his talk, he passed around a container holding a wafer die the size of a thumbnail. On it were 160 or so of the microrobots.
"What we do is slightly change how the wheels turn," he says. "Simple devices with a fairly simple behavior can be engineered to behave slightly different when you apply a mobile control signal. That allows a very complex set of behaviors." The robots contain an actuator called a scratch drive, which bends in response to voltage running through the electric array. When it releases tension, it goes forward, in a movement similar to an inchworm's. But the key to the robots' varying behavior is the arms extending from the actuators. A steering arm on a microrobot snaps down in response to a certain amount of voltage, dragging on the surface and causing the robot to turn. By snapping the arm up and down as many as 600 times a second, the team can control how much a given robot turns. To control a swarm, the team designed each robot with an arm that reacts to a different voltage. Computer algorithms vary the voltage, prompting the robots to move in complex ways.
"Electrostatic robots have the advantage that you supply power through the electric array, and that's all they need," says Gorman. "It can be very compact. It's very possible to have electrostatic microrobots embedded inside other things [like computer chips]. For magnetic robots, you have to supply electromagnetic field, and that requires a lot of hard work." Others have worked on electrostatic microrobots, he adds, but this work is the furthest along.
"His research is very advanced in terms of controlling multiple microrobots," says Zoltan Nagy, a roboticist at ETH Zürich who works with groups of magnetically controlled robots called Magmites.
"Most of the work to date has been on one robot, and you can move it around on a substrate," adds Gorman. "In a way, all of the applications of interest would require lots of robots, like a colony of ants."
So far, Paprotny has been able to control up to 10 robots on a single surface at once, and the robots can move several thousand times their body length per second, as detailed in a paper that is currently under review. His next plan is to adapt the setup for a liquid environment so that the microrobots can assemble components of biological tissue into patterns that mimic nature.
"We're trying to come up with ways of self-assembling little tissue units," says Ali Khademhosseini, an associate professor at Brigham and Women's Hospital at Harvard Medical School and a specialist in tissue engineering who is collaborating with Paprotny. "In the body, tissues are made in a hierarchical way—units repeat themselves over and over to generate larger tissue-like structures." Heart tissue, for example, has regions for blood vessels, while liver tissue has a repeating hexagonal shape.
So far, Khademhosseini has encased cells in jelly-like hydrogels and assembled them (using methods that include liquid-air interactions and surface tensions) into different regions to mimic biological tissues, such as the fiber-like structures of muscles. But he thinks the self-assembling microrobots will allow more control in creating the tissues.
"We can try to combine cells and materials in microfabrication systems to come up with structures and assemble them in particular ways using the techniques Igor has developed," says Khademhosseini.
He envisions fabricating the gels and cells on top of teams of half a dozen or so robots working in parallel to construct different parts of a tissue. "We could use the robots to do assembly," he says. "The cells, once they're assembled, come off from the robots, letting cells rearrange further to make things that are indistinguishable from natural tissue." Initially, he hopes to create small patches of heart tissues, and then things like heart muscles and valves, and assemble them all together in a heart. "That's where things are heading," he says. "But right now the challenge is we're still not very good at making each of these individual components."
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